Seismologists have long predicted that the big one would probably be a repeat of the 1923 Kanto earthquake, which occurred in a dangerous fault zone close to Tokyo and killed an estimated 142,000 people.

Japan is a tectonically complex zone where three major plates, the Pacific plate, the Okhotsk plate, and the Philippine plate are all ramming into each other. (Learn more about tectonic plates.)

The 1923 earthquake—estimated to have had a magnitude of between 7.9 and 8.4—came from the collision between the Philippine plate and the islands of Japan, in a fault zone known as the Sagami Trough, offshore from Tokyo.

Last week's earthquake occurred farther north, in the southern part of the Japan Trench, formed by the collision of the Pacific and Okhotsk plates.

"Most [experts] didn't expect one so big from there," Chris Goldfinger, director of the Active Tectonics and Seafloor Mapping Laboratory at Oregon State University, said by email.

That's because the Japan Trench has always produced big but not monstrous quakes, at least in the thousand years since humans have been keeping track, he said.

Seth Stein, a geophysics professor at Northwestern University in Evanston, Illinois, agreed.

"The Japanese have spent a lot of time preparing for a big earthquake somewhat further south—basically in the Tokyo area," he said.

Not that the Japan Trench had been seismically inactive, experts noted. "Northern Japan has had earthquakes throughout its history," said Robert Yeats, a professor emeritus of geology at Oregon State University in Corvallis, Oregon.

The problem is simply that scientists don't have enough long-term records of past earthquakes to predict the largest possible earthquake from any given location, according to Northwestern's Stein.

For instance, University of Tokyo earthquake specialist Yasutaka Ikeda has made high-precision GPS measurements of the rate at which tectonic forces are compressing the plates along the Japan Trench, according to Oregon State's Yeats.

Ikeda compared those measurements to the rate at which tension has been released by earthquakes.

His conclusion showed "there were insufficient earthquakes in the last century to [ease] all of that strain," Yeats said. "He suggested that there might be larger ones in the future"—such as the March 11 earthquake.

Ikeda was in China at the time of the earthquake, but even he was caught by surprise. "I did not expect that one to occur while I am alive," he told National Geographic News via email.

Japan Earthquake Preceded by Foreshock

Another warning of Friday's earthquake was visible only in hindsight: A magnitude 7.2 earthquake that had struck the same region on March 9.

Normally earthquakes of that size are followed by smaller aftershocks, not bigger earthquakes. (See earthquake pictures.)

"Lots of magnitude 7 earthquakes are not followed by anything larger," USGS's Applegate said. "It's only in hindsight that we can look back and see that it is a foreshock"—as was the case with the March 9 earthquake.

Only 1 time in 20 do such earthquakes produce aftershocks that are larger than the initial temblor, he added.

The bottom line is that "earthquakes don't come with a warning," U.S. Federal Emergency Management Administration administrator Craig Fugate said during the March 11 briefing.

No matter how carefully we build damage-resistant structures, "things like tsunamis and the shaking of [an] earthquake [are] still going to cause damage."